Apparatus and method for 3-D measurement using holographic scanning

ABSTRACT

A 3-D measurement system utilizing a holographic scanner is provided with beam direction compensation means for compensating for changes in angular deflection of the scanner and further means is provided for synchronizing the scanner diffraction segments with the camera imaging mechanism of the 3-D system as well as for providing multiple offsets of the projected beam.

This is a division of application Ser. No. 884,494, filed July 31, 1986,now U.S. Pat. No. 4,758,093.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus and method for three dimensional(3-D) vision sensing or measurement and, in particular, to an apparatusand method for three dimensional vision sensing or measurement utilizingholographic scanning of the object or surface whose coordinates are tobe measured or sensed.

U.S. patent application Ser. No. 697,796, filed and assigned to the sameassignee hereof, discloses a 3-D measurement system in which theprojector of the system utilizes a holographic scanning mechanism totransform the projected radiant energy beam of the projector into acollection of projected light planes. These light planes are directedonto the surface of the object to be measured and a camera mounted at anangle to the projected planes records the images of the intersections ofthe planes and the surface. These images can then be uniquelyinterpreted or processed to provide the 3-D coordinates of everyresolvable point in the camera image of the surface via thetriangulation relationship of the known baseline distance between thecamera and projector, and the known angles of the optical axes of theprojector and camera relative to the baseline.

Holographic scanning mechanisms of the type to be used in themeasurement system of the '796 application are commercially availableand generally are formed as rotatably mounted glass disks having sectorseach provided with grating grooves etched into the surface of the disk.When a radiant energy beam, such as, for example, a laser beam isprojected at an angle through the disk, the beam is deflected by thegratings of the disk. The amount of deflection is dependent upon thegrating spacing, so that by incorporating several sectors havingdifferent grating spacings about the disk, the beam can be deflected atdifferent angles as the beam impinges on each of the sectors of thedisk. Moreover, as the beam encounters a particular sector and isdeflected through the unique deflection angle corresponding to thesector, the rotation of the sector causes the beam to scan a straightline. The overall effect of rotating the disk on the projected beam isthus to create a collection of projected planes.

Advantageously, because of the optical properties of the grating sectorsof the holographic disk, the deflection angle, which is a diffractionphenomenon, is maintained even if the disk wobbles. Off center rotationerrors also do not affect performance, so that by using the holographicdisk, a lower cost measurement system can be realized for a givenprecision requirement. Furthermore, the ability of the holographic diskto provide a deflection change on a segment to segment basis in anypreselectable manner is advantageous over the continuous scanning of abeam brought about by scanning mechanisms formed from continuouslyrotated reflective surfaces or the like.

The utilization of a scanning beam as provided by the holographic diskto provide radiant energy along a plane, moreover, results in a moreuniform intensity of the radiant energy as compared to spreading theenergy over a plane with a lens. This provides a decided advantage whentrying to maximize the dynamic range of the surface reflections overwhich a measurement and vision sensing system will operate.

In addition to disclosing the above discussed holographic measuring andvision sensing system, the '796 application also discloses that thenumber of projection planes realizable by such a system can be increasedby cascading a plurality of scanning mechanisms, e.g., a plurality ofholographic disks or by inserting a refracting material into the path ofthe radiant energy beam after passage through the scanning mechanism. Inthe latter case, the refractor provides a lateral offset to the beam ascompared to the beam without the refractor and, therefore, effectivelydoubles the number of radiant energy planes realizable by the system.

U.S. Pat. No. 4,238,147, also assigned to same assignee hereof,discloses a 3-D measurement system in which the lens plane and filmplane of the camera of the system and the plane of the radiant energyintersect in a common line. This type of configuration allows the camerafocus to be set so that it is in proper focus for substantially allpoints 10 of the surface to be recorded, without having to readjust thesetting of the focus for differently located points on the surface.

While the above prior art systems have been found usable, there is stilla need to provide an overall 3-D measurement system employingholographic scanning which is less costly to manufacture and which doesnot suffer from temperature sensitivity usually attendant holographicscanners. Furthermore, it is important in such a system that synchronismbe maintained between the system components including scanner and camerasystem and that there be provided means for increasing the projectedplanes in an inexpensive and simple way.

It is therefore a primary object of the present invention to provide a3-D measuring system in which holographic scanning is utilized and inwhich the system components are coordinated and arranged to minimizemanufacturing costs, to provide precision and repeatability in thedeflection process, to allow for arbitrary deflection without a loss inperformance and to provide simplicity in the overall system design.

It is a further object of the present invention to provide a 3-Dmeasurement system of the aforesaid type in which a mechanism isincluded for synchronization of the system components.

It is yet a further object of the present invention to providearrangements for increasing the projection plane capacity of such a 3-Dmeasurement system, without adding significantly to the complexity ofthe system.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, the aboveand other objectives are realized, in part, in a 3-D measurement systemin which a holographic scanning mechanism is utilized in the systemprojector and in which the system projector is further provided with abeam direction compensator which compensates for changes in thedeflection angle of the scanner, resulting from various factorsincluding changes in wavelength of the radiant energy beam caused byenvironmental changes such as temperature variations. In addition tosuch beam direction compensator, the system is further provided withsynchronizing means for synchronizing the operation of the scanner withthe camera imagining mechanism of the system.

In the embodiment of the invention to be disclosed hereinafter, theprojector components are mounted on a base plate and include a narrowband source of radiation and beam forming optics for forming a radiantenergy beam therefrom. The beam is passed through the directioncompensator which precedes the holographic scanner. The latter, in turn,is in the form of a rotating disk around whose circumference aresegments of grating patterns. Focusing means and beam offsetting meansfollow the scanner and provide the resultant beam focused to the desiredstandoff distance on the object or surface to be measured. The beam isscanned by rotation of the holographic disk grating segments each ofwhich also provide a particular deflection to the beam to thereby resultin a collection of scanning planes.

The base plate provides a support for all the projector components. Italso keeps the projected energy and the imaged field resulting from thesystem camera imagining system, which is likewise supported on theplate, in a fixed relationship between the time of calibration of thesystem and the time of actual use of the system for measurementpurposes. The camera imaging system includes camera electronics, animaging array and a lens which focuses reflected light from the objectsurface at the desired standoff distance onto the imaging array.

For synchronization of the camera imaging system and the holographicscanner, this embodiment further includes a synchronizing controllerwhich synchronizes the output of the source of radiation which is in theform of a pulsed laser to the line rate of the camera electronics TVformat signal. .The synchronizer also synchronizes the holographicscanning disk so that it rotates to bring a first segment adjacent theradiant energy beam, then is held in this position until the verticalretrace time of the TV format signal and is then rotated at the ratewhich passes each disk segment through the radiant energy beam in eachsuccessive field time of the TV format signal.

In the disclosed embodiment, the projector of the system is alsoprovided with a beam offsetting means in the form of a plate situated ata fixed angle relative to the projected radiant energy beam and whoseposition in and out of the beam path is solenoid controlled. With theoffsetting means in the path of the beam, the beam is offset parallel toits original path. The offsetting means thus effectively doubles thenumber of projected beam paths and, hence, projected planes realizablewith the system.

With the central optical axes of the projected radiant energy beam andthe camera imaging system of the above-described 3-D system forming ahorizontal plane, the radiant energy beam is vertically scanned, formingessentially a vertical plane of radiant energy within one exposureperiod of the imaging array of the camera system. Each grating patternsegment of the holographic disk deflects this vertical light planethrough slightly different horizontal angles forming a collection ofvertical radiant energy planes which may be doubled in number byrotating the disk twice; once with the offsetting means out of the pathof the beam so that no offset occurs and once with the offsetting meansin the path of the beam so that the beam is offset preferably by halfthe distance between the planes.

To measure the coordinates of the points on the surface of an objectwith the above embodiment of the invention, the aforementioned projectedradiant energy planes sequentially confront thin surface stripes on thesurface which are individually recorded by the camera system array andread out by the camera electronics. This camera signal is then convertedby processing electronics into digital signals and the latter are fed toa computing means. The computing means converts the digital signals intomeasurement values indicative of the 3-D coordinates of each irradiatedsurface point on the object. Optical triangulation based upon the knownprojector to camera system separation or distance, the known projectionangle and the measured camera system angles of each image, form thebasis for computing the surface coordinates.

In further aspects of the present invention, the offsetting means of the3-D system is formed by utilizing multiple glass diffraction plates,preferably formed to have a binary sequence of thickness. In analternative construction, the offsetting means is in the form of asingle plate of stepped thickness. In either case, the number ofprojected radiant energy planes is increased in a simple manner,enabling a larger number of object surface points to be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description inconjunction with the following drawings, in which:

FIG. 1a shows a top view of a holographic 3-D measuring system sensor inaccordance with the principles of the present invention;

FIG. 1b shows a side view of the 3-D measuring system sensor of FIG. 1a;

FIG. 2 illustrates a 3-D measuring system incorporating the measuringsystem sensor of FIGS. 1a and 1b.

FIG. 3 illustrates the details of the beam direction compensation meansfor the holographic scanner and the beam offset means of the projectorof the measuring system sensor of FIGS. 1a and 1b;

FIGS. 4a and 4b illustrate a first and second embodiments of beam offsetmeans in accordance with the invention; and

FIG. 5 shows a single TV field or line of imaged data improperlysynchronized.

DETAILED DESCRIPTION

FIG. 1a is a top view of a 3-D holographic measuring system sensor 127in accordance with the principles of the present invention. The sensor127 utilizes a scanned radiant energy beam, shown as light beam 126, toilluminate stripes on the surface of an object 113 whose surface pointsare to be measured. The illuminated stripes are imaged on an imaginingarray 16 of the sensor camera imagining system. Via triangulation of theprojected path 111 and the imaging path 112 at measureable anglesrelative to the fixed baseline between these paths on the supportingbaseplate 10, the 3-D coordinates of the object surface points can bedetermined.

FIG. 1b shows a side view of the sensor 127 with its housing 125 inplace. The housing 125 encloses all components of the sensor, thesecomponents being all mounted on baseplate 10. Openings 125, in thehousing allow passage of light along paths 111, 112, 114, 124, and 125and preferrably are sealed by material that blocks ambient light butreadily passes radiation at the wavelength of the light beam source 123.Light along path 111 is scanned toward lower path 124 within a verticalplane in order to illuminate a vertical stripe on surface 113. Thisvertical scan and subsequent horizontal stepping of light path 111toward path 114 and light path 124 toward path 125 is accomplished via aholographic scanning mechanism comprised of a rotating disk 120 drivenby motor 118. Disk 120 contains holographic grating segments of the typedescribed above and in the above-referenced '796 application.

Light beam source 123 forms light beam 126 via a solid state laser diodeprovided with beam forming optics. Light beam 126 is reflected by arotatably mounted mirror 122. Mirror 122 may be rotated about a verticalaxis to provide a horizontal angular adjustment during alignment of thedirection of light beam path 111. Light beam-126., after reflection bymirror 122, is diffracted (bent) approximately 90 degrees by beamdirection compensator 121, which is also adjustable about a verticalaxis to assist in alignment. Light beam 126, after diffraction bycompensator 121, is again diffracted approximately 90 degrees in theopposite direction by a grating segment of holographic disk 120.Thereafter, diffracted light beam 126 is focused by lens 119 along lightpath 111 to a minimum width at a specified distance from the sensor 127.This distance is defined as and referred to herein as the nominalstandoff distance. A solenoid 117, when activated, inserts a glass plate115 into the light beam paths 111-114 in the direction 116. When lightbeam 126 passes through glass plate 115, which is at an angle to thelight beam path, the light beam is laterally offset as discussed aboveand in the '796 application. By adjusting the angle of the plate 115offsets approximately equal to one-half the distances between the pathsat the nominal standoff distance may be obtained.

Each grating segment on disk 120 scans light beam 126 vertically anddiffracts the beam at slightly different angles between light beam paths111 and 114. The angles are selected to equally space the paths. Whenthe plate 115 is utilized the number of light beam paths is doubled.

All the elements 123, 122, 121, 120, 119, 118, 117, 115 mounted on plate11 taken together collectively form the projector of the sensor 127.Plate 11 can be rotated about a vertical axis during the time ofalignment to provide a variety of standoff distances and off centermeasurements. The directions of rotation are indicated by arrows 110 inFIG. 1a.

The remaining elements, (i.e., surface 15, mirror 14, lens 13, cameraelectronics 12, imaging array 16 and surface 17), form the cameraimagining system of the sensor and are also directly mounted tobaseplate 10. Light reflected from stripes of light on surface 113illuminated by the projector is collected within light beam path 112,reflected off mirror 14, imaged by lens 13 upon imaging array 16. Cameraelectronics 12 reads out the image from array 16 and generates astandard TV format signal.

Mirror 14 is mounted on the surface 15 which is rotatable about avertical axis. During alignment, the surface 15 is rotated to aim lightbeam path 112 to cross the projected light beam paths 111, 124, 114, 125which form the four corners of the volume within which the projectedlight beam is confined. Path 112 is likewise a volume whose crosssectional area is imaged onto array 16. Surface 15 and, therefore,mirror 14, are rotated in directions 19 to aim light beam path 112 tocenter the common volume of path 112 and projected light beam paths 111,124, 114, 125 at the nominal standoff distance. Lens 13 is focused forsharpest image at the nominal standoff distance and stopped down toprovide adequate depth of field. Imaging array 16 is mounted on thesurface 17 which is rotatable about a vertical axis as indicated bydirections 18. Surface 17 and array 16 are rotated to obtain thesharpest average image of the projected light beam 126 along paths 111,124, 114, 125 following the procedure of the '147 patent.

FIG. 2 illustrates a vision sensor 227 (the components of the visionsensor 227 corresponding to those of the vision sensor 127 shown in FIG.1 have the same last two digits, e.g., solenoid 217 in FIG. 2corresponds to solenoid 117 in FIG. 1), incorporated into a complete 3-Dmeasuring system. Computer 230 initiates a measurement via a requestsignal 236 to controller 228. Controller 228 provides all the timing andsynchronization required for the system. Vertical and horizontalsynchronizing signals 235 are provided by the controller to cameraelectronics 22, stepping pulses 233 are provided to motor 218, anactivation signal 234 is provided to solenoid 217, a laser pulsetriggering signal 232 is provided to light beam source 223 and timingpulses 237 are provided to processor 229. Processor 229 also receives avideo or TV format signal 231 from camera electronics 22 and reports viasignal 238 to computer 230 the TV line number and centroid (quantizationcount along the horizontal TV line) representing the light stripe imagelocation on each TV line. Computer 230 converts the reported TV linenumbers and centroids to 3-D coordinate data using calibration datastored in memory. The 3-D coordinate data are outputted via signal 239from computer 230.

FIG. 3 illustrates the details of the projector of vision sensor 127(again, in FIG. 3, like components as those in FIG. 1 have the same lasttwo digits). Light beam 326 is incident on beam direction compensator321 at angle 340 and exits at angle 341 after being diffracted by adiffraction grating on compensator 321. Angles 340 and 341 are relatedas follows: ##EQU1##

Likewise, light beam 326 is incident on holographic disk 320 at angle342 and exits at angle 343 after being diffracted by the diffractiongrating of the disk. The angles 342 and 343 are related as follows:

    sin (angle 342)+sin (angle 343)=λ/d

where λ and d are as defined above

Optimal performance of the sensor is obtained when angles 340, 341, 342,343 are 45 degrees. Sensitivity to disk wobble is minimized since anincrease in angle 342 is matched by a decrease in angle 343. Also, asenvironmental conditions such as temperature tend to affect thewavelength, λ, of the light beam source 123, angles 341 and 342 willchange by equivalent amounts, thus effectively cancelling environmentalsensitivity. This is assured by selecting the diffraction gratingspacing d of the compensator 321 to be similar to that of the gratingson the disk segments.

As can be appreciated, therefore, the use of beam direction compensator321 situated at the same angular orientation as holographic disk 320 andhaving a similar diffraction grating characteristic as the disksegments, provides compensation for changes in angular deflection of thedisk 320 due both to mechanical wobble as well as environmental changes.The overall effect is thus a more reliable and accurate sensor.

After light beam 326 passes through disk 320 it is focused by lens 319and the focused beam is projected along path 346. When glass plate 315is interposed into path 346, however, beam 326 is incident on plate 315at an angle 344 and is offset along path 345 parallel to path 346. Thesurfaces of plate 315 are parallel and the offset is given by:

    offset=[T sin (θ.sub.1 -θ.sub.2)]/ cos θ.sub.2

where

T=plate 315 thickness

θ₁ =angle 344

θ₂ =arc sin [n1 (sin θ₁) /n2]

n1=index of refraction of air=1

n2=index of refraction of glass=1.5

As disk 320 is rotated about axis 347, each grating segment scans beam326 in a plane perpendicular to the plane of the drawing along a path346 (or 345) at different angles 343 as determined by grating spacing dof the segment.

While the plate 315 allows for an offset beam path 345 in the projector,it would be desirable to be able to obtain an even larger number ofoffset paths while not over-complicating the projector structure. FIG.4a illustrates a plate configuration which permits such a larger numberof offset beam paths to be realized with a minimum of separate glassplates.

As shown in FIG. 4a, incident light beam 42 normally would follow path43. Interposing glass plate 41 along direction 47 in path 42 wouldoffset path 43 to 414. Interposing just glass plate 40 along direction46 into path 42 would offset path 43 to 44 and interposing both glassplates 40 and 41 would offset path 43 to path 45. By choosing thethickness of plate 40 to be twice that of plate 41, three equally spacedoffset paths can be generated with two glass plates. In general, byusing N plates whose successive thicknesses increase 2:1, 2^(N) -1offset paths can be generated.

FIG. 4b shows an alternative configuration for realizing multiple offsetpaths. In this case, a single glass plate with stepped thicknesses isused. Incident beam 410 would normally follow path 417. Interposingfirst thickness step 411 of glass plate 48 along direction 49 offsetspath 417 to path 416. Further motion of glass plate 48 interposes secondstep 412 which offsets path 417 to path 415. By interposing the finalstep 413 the path 417 is offset to path 414. Equal offset increments areobtained with equal increment thickness steps.

FIG. 5 illustrates the image that may be read out of the imaging array16 of sensor 127 when the rotation of disk 120 is not properlysynchronized with the array 16 readout. For proper synchronization therotation rate must be selected to rotate each grating segment on disk120 through beam 126 in the time of one TV field of the TV format signalgenerated by the camera electronics. Each grating segment on disk 120subtends an equal angle so that continued synchronization is possible.To this end, the phase angle of the rotation of disk 120 is adjusted sothat the light beam 126 transitions from one segment to the next duringthe vertical retrace time of the TV format signal. By transitioningprior to the end of the vertical retrace time by at least five times thetime it takes to readout a single TV line it can be assured forreasonably well focused spot sizes that the scanning spot ofillumination provided by light beam 126 on surface 113 will not degradethe output video TV format signal 231.

Looking now at FIG. 5 and assuming the scanning spot of beam 126 movesfrom top to bottom on the image and readout is from top to bottom, thenif imaging array 16 transfers the image at the vertical retrace time ofthe TV format signal, readout field 50 will contain the broken image 51,52 if the scanner disk 120 is not synchronized with the array readout.In such cases, light beam 126 is scanned by a portion of one segment ofdisc 120 forming image 52 and is then scanned by the next segment ondisc 120 forming image 51. This is corrected by synchronizing segmenttransitions to occur during vertical retrace times of the TV formatsignal as previously described.

Synchronization may either be effected with disk 120 continuouslyrotating or by starting rotation on command. If continuous rotation ispreferred then, referring to FIG. 2, when computer 230 requests ameasurement, controller 228 must either wait for the segment on disk 120that represents the start of the sequence of segments or processor 229must be capable of starting with any segment in the sequence.Alternately, by using an accurately controllable motor 118, such as astepping motor, disk 120 may be positioned such that the beam 126 isslightly before the starting segment and on request for measurement, thecontroller need only wait for the next vertical retrace time, acceleratedisk 120 to speed, and processor 229 will receive the data in a fixedsequence. This also enables stopping disk 120 after one revolution,interposing glass plate 115 and repeating the process with offset lightbeams. When using continuous rotation of disk 120, either the disk mustcomplete one full revolution during which time glass plate 115 is movedinto place, or processor 229 must be capable of starting up within thesequence after glass plate 115 is in place.

In all cases, it is understood that the above identified arrangementsare merely illustrative of the many possible specific embodiments whichrepresent applications of the present invention. Numerous and variedother arrangements can readily be devised in accordance with theprinciples of the present invention without departing from the spiritand scope of the invention. Thus, for example, a third alternative toobtaining multiple offset paths for the beam 326 of FIG. 3 is to alterthe angle 344 by rotating plate 315 about the vertical axis.

What is claimed is:
 1. A method of offsetting the path of a radiantenergy beam to multiple paths with a relatively few glass platescomprising: inserting a first glass plate at a predetermined angle intosaid radiant energy beam, to offset said beam to a first offset path;removing said first glass plate and inserting a second glass plate atsaid predetermined angle into said beam to offset said beam to a secondoffset path, said second glass plate being twice as thick as said firstglass plate; and inserting said first glass plate with said second glassplate at said predetermined angle into said beam to offset said beam toa third offset path.
 2. A method in accordance with claim 1 furthercomprising:continuing said method for N glass plates, with eachsuccessive plate being twice as thick as the immediately preceding plateto offset said beam to 2^(N) -1 offset paths.
 3. A method of offsettingthe path of a radiant energy beam to multiple paths with a single glassplate having at least one step thickness change comprising: interposingby translating at a predetermined angle the thinnest step of said plateinto said beam to offset said beam to a first offset path; translatingsaid glass plate at said predetermined angle to bring the next thinneststep of said plate into said beam to offset said beam to a second offsetpath.
 4. A method in accordance with claim 3 further comprising:saidplate having N steps; and continuing said method to translate all Nsteps into said beam to offset said beam to N offset paths.